MIC24052 Datasheet, PDF (24/34 Pages) Micrel Semiconductor – 12V, 6A High-Efficiency Buck Regulator

MIC24052
EQUATION 5-17:
K_DIV = -_R---_I--_N---^R-_J---1+----/-/_R--R--1--2--/-/--R----2-
In Equation 5-16 and Equation 5-17, it is assumed that
the time constant associated with C_ff must be much
greater than the switching period:
EQUATION 5-18:
--------1-------- = ^T-- « 1
f_SW   
If the voltage divider resistors R1 and R2 are in the kΩ
range, a C_ff of 1 nF to 100 nF can easily satisfy the
large time constant requirements. Also, a 100 nF
injection capacitor C_INJ is used in order to be
considered as short for a wide range of the
frequencies.
The process of sizing the ripple injection resistor and
capacitors is:
1. Select C_ff to feed all output ripples into the feed-
back pin and make sure the large time constant
assumption is satisfied. Typical choice of C_ff is
1 nF to 100 nF if R1 and R2 are in kΩ range.
2. Select R_INJ according to the expected feedback
voltage ripple using Equation 5-19.
EQUATION 5-19:
K_DIV
=
_----V_{----F---B-----P----P---}
V_IN

-_D-----^f_--^S---^W_---1--^--–---^--_D----_-
Then the value of R_INJ is obtained as:
EQUATION 5-20:
R_INJ
=

R1
//R2




-----1-------
K_DIV
–
1_
3. Select C_INJ as 100 nF, which could be consid-
ered as short for a wide range of the frequen-
cies.
5.5 Setting Output Voltage
The MIC24052 requires two resistors to set the output
voltage as shown in Figure 5-4.
The output voltage is determined by Equation 5-21:
EQUATION 5-21:
V_OUT
=
V_F
B



1
+
RR_{-----12-}
V_FB equals 0.8V. A typical value of R1 can be between
3 kΩ and 10 kΩ. If R1 is too large, it may allow noise to
be introduced into the voltage feedback loop. If R1 is
too small, it will decrease the efficiency of the power
supply, especially at light loads. Once R1 is selected,
R2 can be calculated using:
EQUATION 5-22:
R2 = _V----^V_O----^FU---^B-_T---^-–----^RV----1_F----_B-
FIGURE 5-4:
Configuration.
Voltage Divider
In addition to the external ripple injection added at the
FB pin, internal ripple injection is added at the inverting
input of the comparator inside the MIC24052, as shown
in Figure 5-5. The inverting input voltage V_INJ is
clamped to 1.2V. As V_OUT is increased, the swing of
V_INJ will be clamped. The clamped V_INJ reduces the
line regulation because it is reflected as a DC error on
the FB terminal. Therefore, the maximum output
voltage of the MIC24052 should be limited to 5.5V to
avoid this problem.
DS20005659A-page 24
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